use newtypes for Password and Identity to avoid usage errors

This a breaking API change. The next release should bump the minor version
number.

As discussed in https://github.com/warner/spake2.rs/issues/3 and
https://github.com/warner/magic-wormhole.rs/issues/32 , if an application
were to accidentally swap the "password" and "identity" arguments (mainly for
start_symmetric which only takes two args), the app would appear to work, but
would contain a devastating security vulnerability (online brute-force
password attack, with precomputation enabled).

You might think of newtypes as giving the API named parameters. Instead of:

`s = start_symmetric(b"pw", b"appid")`

you get:

`s = start_symmetric(&Password::new(b"pw"), &Identity::new(b"appid"))`

but it protects you (with a compile-time error) against mistakes like:

`s = start_symmetric(&Identity::new(b"appid"), &Password::new(b"pw"))`

I'd like to find a way to remove requirement to pass a reference (and enable
`start_symmetric(Password::new(..)..)`).

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